The present invention relates to a process for the production of azo macroinitiators for the synthesis of block copolymers and for the production of aqueous block copolymer dispersions and emulsions, and to uses thereof.
Polymers are made from monomers or from other polymers by appropriate reactions. According to the IUPAC, in these so-called polyreactions, a distinction is made between polymerization and polycondensation. Polycondensation is characterized by a repeated condensation process in the course of which simpler molecules are eliminated. For example, many polyamides are prepared from diamines and dicarboxylic acids by this process with elimination of water molecules.
On the other hand, during polymerization, continuing addition of molecules to one another takes place, without simultaneous elimination of another molecule. These reactions are generally triggered by so-called polymerization initiators, which can be of an anionic, cationic, or radical nature.
Although the nature of the monomers is of decisive importance for the selection of the initiator, nonetheless, if possible, preference is given to radical polymerization.
The production of block copolymers also takes place by radical polymerization. A survey of the various methods of synthesis is, for example given, in the "Encyclopaedia of Engineering Materials, Part A, Polymer Science and Technology, Volume 1, Part 4, Synthesis and Characterization of Block Copolymers" (Marcel Dekker Inc. 1988). Of the various detailed papers, in particular those of J. Furukawa (Agnew. Makrom. Chem. 1 (1967), 92); S. Nagai (J. Polym. Sci., Part A 24 (1986), 405); J. Piirma (J. Appl. Polym. Sci. 26 (1981), 3103); as well as that of B. Hazer (Agrew, Makrom. Chem. 129 (1985), 31), may be mentioned.
The block copolymers made by the methods described, particularly those based on urethane and acrylic polymers, have outstanding application technology properties. Preferred use fields are therefore surface coatings and varnishes (see Japanese Specification First Publication Hei 3-74420).
However, a problem in the production and use of block copolymers is the use of non-aqueous solvent systems, which must be regarded as problematical in relation to environmental and industrial hygiene. For this reason, the present trend is towards the use of aqueous polymer dispersions or emulsions.
However, until now, very few synthetic methods for block copolymers in aqueous dispersion have been developed. Thus, D. Mikulasova (Makrom. Chem. 175 (1974), 2091) describes a method for the synthesis of vinyl and vinyl block copolymers in emulsion using long-lived radicals. Similarly, I. Piirma (J. Appl. Polym. Sci. 33 (1987), 727) describes a synthesis of vinyl and vinyl block copolymers in a two-stage radical polymerization using azo and peroxo radical starters. Block copolymers of polyvinyl chloride and polyethylene oxide were synthesized by J. J. Laverty (J. Polym. Sci., Polym. Chem. Ed. 15 (1977), 2001) and by S. Nagai (Chemistry Express 1 (9), (1986), 543) using polymethylene oxide with azo groups. G. Smets (J. Polym. Sci., Polym. Chem. Ed. 16 (1978), 3077) uses polymeric peroxides as initiators for the synthesis of block copolymers. In this context, reference should also be made to an emulsion polymerization in which surfactant polymeric azo initiators serve as emulsifiers (W. Heitz (Makrom. Chem. Rap. Commun. 2 (1981), 83)).
Emulsion block copolymers are gaining ever-increasing importance in the field of surface coatings, especially as paints, as can for example clearly be seen from JP First Publication HEI 1-254780, which describes the use of block copolymers of polyethylene glycols and polyacrylates as paints.
However, for the present, as can be seen from the known state of technology, the markedly increased interest in emulsion block copolymers cannot yet be satisfactorily accommodated, since their production requires a difficult and costly production process.
Heretofore, conventional procedures for the production of the azo macroinitiators as a starting component for the production of aqueous dispersions and emulsions of block copolymers are characterized in that:
A) All components are added dropwise simultaneously, or
B) Isocyanate-terminal polymers are prepared from a polymer with functional terminal groups by reaction with a diisocyanate, and this then reacts with suitable stoichiometric amounts of an azo alcohol.
Azo macroinitiators which are not very homogeneous in their molecular composition are obtained.